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Abstract

Different designs for producing multiple stopband mesoporous silicon rugate filters via electrochemical anodization are compared. The effects of light absorption and dispersion to visible range filter design are investigated. Thermal oxidation is applied for passivating the chemically reactive porous silicon surface, and the response of the passivated structures to ethanol vapor is examined. Differences in gas sensing properties for the various designs are evaluated and possible reasons for the observed differences are discussed. Methods for sidelobe suppression in multipeak filters are discussed and demonstrated, and their effects in gas sensing applications are estimated.

Figures (11)

Reflectance spectra for two PS rugate filters prepared with a sinusoidal anodization current oscillating between 49.4 and 89.9 mA/cm2. The period durations were (a) 3 s and (b) 5 s, with the total preparation times being 180 and 200 s, respectively.

Reflectance spectra for two distinct dual-peak PS rugate filters prepared by stacking two filters similar to the ones shown in Fig. 1 on top of each other. (a) A filter prepared with similar parameters as the one shown in Fig. 1 (a) is on top, and a filter shown in Fig. 1 (b) is at the bottom. (b) The stacking order for the filters is reversed. The reflective peak originating from the bottom filter is completely invisible due to light absorption at the lower wavelength range. Existence of the lower filter only manifests irregularities in the sidelobe fringe pattern.

Imaginary part of the porous silicon refractive index determined for an as-anodized sample produced with 88.6 mA/cm2 anodization current density. The extinction coefficient for lower wavelengths increases considerably. This behaviour is analogous to crystalline silicon refractive index.

Rugate filters displaying two reflective peaks produced with different current profiles ((a) linear combination of two sine waves, (b) successive sine waves with different periodicity, (c) successive sine waves with different periodicity and offset level). The insets display schematic profiles for the anodization current density J. Solid and dashed lines represent the measured reflectance spectra before and after thermal oxidation, respectively.

Three single-line rugate filters with (a) normal sinusoidal profile, (b) sinusoidal profile with Gaussian apodization, and (c) sinusoidal profile with Gaussian apodization and quintic refractive index matching layers incorporated at the beginning and end of the anodization current profile.

Response of stacked PS rugate filters to 8012 ± 172 ppm of ethanol vapor. The incorporation of quintic refractive index matching layers does not affect the response time and sensitivity of the porous structure.

Tables (2)

Table 2 Anodization parameters for a superimposed triple-peak PS rugate filter. The offset current density was 88.6 mA/cm2 with the total anodization time being 240 s. The reflectance spectrum for a filter obtained with these parameters is shown in Fig. 8 (b).

1 Offset current density for the superimposed sine waves is defined as shown by equation 2 and therefore the value for both sine waves has to be the same2 Since the sine waves are superimposed their duration equals the total duration of the entire anodization period

Table 2

Anodization parameters for a superimposed triple-peak PS rugate filter. The offset current density was 88.6 mA/cm2 with the total anodization time being 240 s. The reflectance spectrum for a filter obtained with these parameters is shown in Fig. 8 (b).

1 Offset current density for the superimposed sine waves is defined as shown by equation 2 and therefore the value for both sine waves has to be the same2 Since the sine waves are superimposed their duration equals the total duration of the entire anodization period

Table 2

Anodization parameters for a superimposed triple-peak PS rugate filter. The offset current density was 88.6 mA/cm2 with the total anodization time being 240 s. The reflectance spectrum for a filter obtained with these parameters is shown in Fig. 8 (b).